1. Field of the Invention
The invention relates to radar detection of moving targets and more specifically to Doppler radar detection of moving targets in the presence of strong scattering echoes.
2. Description of Related Art
Radar systems transmit radiation toward moving targets and receive the reflected signals from the targets. A phased array having a plurality of sensors distributed over a surface detects the reflected signals from the moving targets. Phased array geometries can be either one or two dimensional. Spatial selection is obtained by concentrating the transmitted and/or received energy to a specific solid angle with reference to the phased array. Focusing is accomplished by adjusting the complex weightings of the signals received at the different receiving elements of the phased array. For a pulsed system, echoes from objects within a range interval also within the focused solid angle, known as a range cell, may be selected by sampling the focused signal at delay times commensurate with the round trip travel time of the pulse to the specific range of interest.
In most state of the art operational radars, moving targets are detected using the Doppler frequency shifts of the radar echoes from the moving targets. The Doppler frequency shifts are proportional to a component of the target's velocity in the direction of an axis from the phased array to the target.
The detection of echoes with small Doppler shifts is limited by two effects: the Rayleigh resolution of the linear spectral analysis, and the spectral spreading due to decorrelation of clutter echoes. The Rayleigh resolution is inversely proportional to the coherent dwell time, that is the time duration of the coherent radar data collection. The spectral spreading of the clutter is driven in part by random scattering of radar echoes off of ocean waves known as "clutter", whose velocity may vary slightly over the many waves within a range cell. For sea clutter under some conditions the spectral spread may be much smaller for a very short coherent dwell than for a longer dwell.
Moving target detection has been accomplished by linear spectral analysis processing methods. Most common among these is the "Moving Target Indicator" (MTI) as described in Introduction to Radar Systems, 2nd Edition, by M. I. Skolnik, pp. 101-106, McGraw-Hill Pub. Co., New York, N.Y. (1980). This method subtracts corresponding echoes measured at consecutive pulse times, thus canceling those echoes whose phase and amplitude do not change from one pulse time to the next. The MTI method is often supplemented by Fast Fourier Transform (FFT) processing, acting on corresponding echoes from several identical, equally-spaced pulses. The FFT processing sorts the echoes into several Doppler frequency ranges known as "Doppler bins", and provides signal to noise enhancement through coherent integration. FFT processing is sometimes used without MTI, but has the disadvantage that large amounts of clutter may appear in the target-associated Doppler bins through a phenomenon known as spectral leakage.
Problems relating to temporal frequency estimation of signals using a sampled time series when temporal coherence is maintained over a time period are conceptually similar to problems of spatial frequency estimation for distant spatially separated sources. Significant advances in spatial frequency estimation have been made in recent years in the development of analyzers employing phased arrays which can accurately estimate the direction of arrival (DOA) of targets.
Electromagnetic or acoustic waves arising from distant targets behave as plane waves incident upon the array. Near optimal performance results for the DOA estimates are exhibited by the subspace array processing algorithms such as, for example, the MUSIC algorithm (See "Multiple Emitter Location and Signal Parameter Estimation", R. O. Schmidt, IEEE Trans Antennas and Propagation, Vol. AP-34, pp. 276-280, March 1986), the Root-MUSIC algorithm, (See "The Mathematical Basis for Element and Fourier Beam Space MUSIC and Root-MUSIC Algorithms", S. D. Silverstein and M. D. Zoltowski, Digital Signal Processing, Vol. 1 pp. 161-175, July 1991), and the ESPRIT algorithm (See "ESPRIT--A Subspace Rotational Approach to Signal Parameter Estimation", R. Roy, A. Paulraj, and T. Kailath, IEEE Trans, Acoust. Speech Signal Process vol. ASSP-34, pp. 1340-1342, October 1986).
Conventional radar Doppler systems compute the Doppler shifts of moving targets but there are problems estimating Doppler shifts for: targets with small radar cross sections; slowly moving targets such as small boats; and slowly moving aircraft in the presence of clutter due to ocean waves.
Currently, there is a need for a moving target detection system that can detect and track moving targets represented by small radar echoes whose Doppler shifts are close to zero, in the presence of much more powerful clutter echoes whose Doppler shifts are also very close to zero.